1. Field of the Invention
The present invention relates generally to a semi-closed circuit passive gas addition breathing apparatus and more particularly to a variable volume ratio compound counterlung used in a rebreathing apparatus.
2. Description of Related Art
Conventional semi-closed rebreathers operate by delivering a premixed gas from a scuba cylinder through a constant flow regulating device, usually by supplying a regulated gas supply to a changeable orifice. Gas is delivered at a preset rate regardless of depth. The gas being breathed is recirculated, and as the oxygen within the mixture is metabolically consumed, it is hopefully being adequately replaced on a continuous basis with a predetermined continuous flow of oxygen enriched gas.
Rebreathers consist of a breathing loop from which the diver inhales and into which the diver exhales. As most of the exhaled gas stays in the breathing loop, rebreathers allow for much greater gas efficiency than open circuit systems. This greater gas efficiency allows for longer duration dives as compared to open circuit systems, or, conversely, requires less gas supply for a dive of equal duration.
The breathing loop generally includes a relief valve, scrubber, counterlung, depth equalization regulator, continuous injection system, hoses and a mouthpiece. The relief valve is utilized for dumping or venting excess gas in the breathing loop created by the rebreather on ascent and excess gas which is produced with the use of constant (active) addition systems. The scrubber cleanses the exhaled gas of carbon dioxide. The counterlung or breathing bag allows for the retention of the diver""s exhalation gas. The injection system adds fresh gas to the carbon dioxide cleansed gas in the breathing loop. The depth equalization regulator adds supply mix to the loop to keep pace with depth increases. The hoses are utilized to connect the counterlung and scrubber with the mouthpiece. The mouthpiece is connected to the two hoses and is the point on the breathing loop where the diver exhales and inhales. Typically, two conventional one-way valves are incorporated into the mouthpiece.
Rebreathers normally include a harness to strap the unit to the diver, with some units also including a protective case for the various above described components.
As stated above, rebreathers generally work by recycling most of a diver""s exhaled breath, which travels through the breathing loop through the scrubber, and is returned to the diver during inhalation. The use of a rebreather allows a diver to remain underwater for a relatively long time as compared to the use of open circuit equipment.
Accordingly, rebreathers allow exhaled gas to be cleansed of carbon dioxide and replenished with fresh oxygen for further consumption. A traditional fixed flow (active addition) semi-closed rebreather recycles the gas the diver is breathing, removing excess carbon dioxide from the exhaled gas and replacing it with a measured amount of premixed gas to maintain an oxygen partial pressure in the inspired gas that will continue to support metabolism.
There are several previously known types of operating systems for semi-closed circuit rebreathers, including fixed discharge ratio, continuous injection and mechanically pulsed. In the 1970""s, as electronically controlled rebreathers were coming into their own, a fixed discharge ratio counterlung (an inner bellows within an outer bellows) was developed for semi-closed use in Europe. This type of rebreather was coined the first xe2x80x9cpassivexe2x80x9d addition or counter mass ratio system. xe2x80x9cPassivexe2x80x9d means gas is only added as required to replace gas that has been discharged from the breathing loop by the control mechanism.
The xe2x80x9cpassivexe2x80x9d addition system discharged a fixed percentage of each exhalation overboard, thus responding to respiratory minute volume (xe2x80x9cRMVxe2x80x9d) or work rate. As such, reasonably tight decompression schedules could be computed for semi-closed equipment, eliminating the need for complex electronic oxygen monitoring.
Any system keyed to RMV is essentially using the diver as a sensor. The passive system uses a proportional discharge valve or a bellows within a bellows to discharge a fixed percentage of every exhalation overboard. The missing part of the exhalation is made up xe2x80x9cpassivelyxe2x80x9d by one or two demand regulators on the following inhalation. Excess gas in the breathing loop from reduced ambient pressure is vented off by an overpressure relief valve. The fixed discharged ratio units maintain reasonably steady oxygen fractions in the breathing loop. The counterlung does not have to be purged on normal ascents to prevent hypoxia.
One drawback with the fixed discharged ratio semi-closed circuit is that it is not as gas efficient as electronic closed circuit rebreathers or constant flow (active) semi-closed rebreathers due to the fact that gas usage increased with depth similar to open circuit equipment. Furthermore, different diver positions often caused gas to be lost. The increased gas usage limits dive duration at depth as compared to other types of semi-closed units. Thus, despite solving decompression problems the bellow within a bellow system was ultimately abandoned due to its limited dive duration capabilities.
The continuous injection system is an active addition system and typically bleeds a fixed flow of single source mixed gas into the breathing loop from a variable or changeable fixed orifice. The flow rate is determined by estimating the diver""s work rate for the intended dive and hopefully ensuring that enough oxygen from the mixed gas supply enters the system to meet anticipated metabolic requirements. Hypoxia is possible if the counterlung is not purged during ascent. Additionally, extended periods of higher than anticipated work loads can also produce hypoxia.
The mechanically pulsed semi-closed rebreather is also an active system and uses a bellows counterlung to mechanically drive a ratchet/cam that pulses gas addition valves in approximate response to respiratory minute volume. The gas addition is from a single mixed gas supply and is regulated to provide reasonably tight oxygen fractions in the breathing loop. Excess gas in the breathing loop from additions or reduced ambient pressure is vented off by an overpressure relief valve. However, with this type of unit, there are more single point addition failure possibilities.
Accordingly, no prior RMV controlled recirculating breathing system has incorporated a mass-constant discharge capability. Thus, there exists a need for a xe2x80x9cpassivexe2x80x9d gas addition semi-closed circuit rebreather unit which provides for a variable discharge ratio which changes with depth to effect a mass constant discharge ratio (to reduce gas wastage) that is controlled by the diver""s RMV (to make the unit responsive to actual metabolic requirements). It is therefore, to the effective resolution of the aforementioned problems and shortcomings that the present invention is directed.
The present invention provides a variable volume discharge ratio compound counterlung for use with a semi-closed circuit breathing apparatus. The entire breathing apparatus incorporating the compound counterlung provides for a variable discharge ratio semi-closed circuit rebreather unit which does not reduce in gas usage efficiency with depth. The compound counterlung consist of a variable volume discharge inner counterlung driven by and disposed within a weighted bellows (outer counterlung). The inner counterlung geometry is chosen such that there is always provided enough discharge capacity to exceed metabolic addition requirements, regardless of depth. The inner counterlung component arrangement takes advantage of both outer counterlung forces and exhalation pressures to ensure accurate volumetric sizing.
The inner counterlung reduces in volume with depth increase, allowing it to discharge exhalation gas inversely proportional to depth. As such, the same amount of mass is always discharged for any given RMV regardless of depth. The shortfall in the diver""s subsequent inhalation is made by conventional redundant addition regulators, associated with the breathing loop. Addition is made when there is zero counterlung volume, thus reducing the gas in the breathing loop that is available to dilute the addition. The other components, which normally make up a rebreather, i.e. canister, scrubber, mouth piece, hoses, etc. can be conventional.
The system is keyed to respiratory minute volume, and makes a full and proportional oxygen correction with every breath. Accordingly, the system is reliable for holding steady inspired oxygen fractions, thus making use of standard programmable decompression computers and hard tables practical.
The variable discharge ratio makes the ejected portion of every breath mass constant relative to the tidal volume and breathing frequency, regardless of depth. Thus, the unit achieves an equal reclaim rate at depth as at surface. As such, the unit is five (5) times more efficient in gas use than an open circuit unit at the surface, and is twenty (20) times more efficient than an open circuit unit at 4 absolute atmospheres (99FSW).
Furthermore, as the present invention discharges part of every exhalation, loss of gas addition results in subsequently shorter volumes of gas available for each inhalation, making an addition failure immediately recognizable and hypoxia highly unlikely. Rational arrangement of the components in the breathing loop make other malfunctions immediately recognizable through other changes in breathing characteristics.
The volumetric capacity of the variable volume inner counterlung is controlled by one or more ambient pressure sensing devices. The depth sensing devices allow for pressure preloading to change the rate of inner counterlung volumetric changes relative to ambient pressures. Sensing device pressure envelopes are also provided that act as indicators for surface pressure registration, inner counterlung floods and bacterial growth. The depth sensing envelopes also allow for leak testing of the inner counterlung.
The outer bellows-type counterlung drives the inner counterlung""s contents overboard with each breathing cycle. The inner bag control arrangement works in a plane perpendicular to the discharge driving motion, thus allowing for volumetric changes that do not affect the range of collapsing motion during the discharge cycle.
The present invention compound counterlung provides for semi-closed cycle passive gas addition for recirculating diver breathing systems that is keyed to both respiratory minute volume and depth by making each discharge mass constant relative to the volumetric relationship of the inner and outer counterlungs at the surface, thus, allowing for superior gas efficiency as compared to prior designs.
A variable volume ratio compound counterlung is provided for use with a semi-closed circuit breathing apparatus. The compound counterlung generally comprises a flexible bag member disposed within an outer counterlung member. The flexible bag member and the outer counterlung member are in communication with an exhaled gas area of a breathing loop. The flexible bag member having first and second ends which are attached to said outer counterlung. A pair of depth sensors operatively associated with the flexible bag member are provided to vary the volume of said flexible bag member with changes in depth. The flexible bag member is driven by the outer counterlung to discharge gas stored within the flexible bag member depending on the diver""s respiratory minute volume. The collapsing of said outer counterlung member also returns gas stored within the outer counterlung back into the breathing loop. The volumetric capacity of the inner counterlung is controlled by one or more ambient pressure sensing devices with an outer bellows-type counterlung that drives the inner counterlung""s contents overboard with each breathing cycle. The invention provides semi-closed cycle passive gas addition for recirculating diver breathing systems that is keyed to both respiratory minute volume and depth by making each discharge mass constant relative to the volumetric relationship of the inner and outer counterlungs at the surface, thus making the system far more gas efficient than previous designs.
Some of the features of the present invention include, but are not limited to, the following:
(1) Depth sensing devices that allow for pressure preloading to change the rate of inner counterlung volumetric changes relative to ambient pressures;
(2) An inner bag control arrangement that works in a plane perpendicular to the discharge driving motion, thus allowing for volumetric changes that do not affect the range of collapsing motion during the discharge cycle;
(3) Depth sensing device pressure envelopes that act as indicators, for surface pressure registration, counterlung floods and bacterial growth;
(4) Inner counterlung geometry that always provides enough discharge capacity to exceed metabolic addition requirements, regardless of depth;
(5) Inner counterlung component arrangement that takes advantage of both outer counterlung forces and exhalation pressures to insure accurate volumetric sizing; and
(6) A discharge control valve that prevents any discharge during the fill (exhalation) cycle to insure accurate volumetric sizing of the inner counterlung under pressure.
Some of the benefits of the present invention include, but are not limited to, the following:
(1) Provides the most efficient use of gas possible in a system that is keyed to RMV while still maintaining the tight inspired oxygen fractions associated with passive addition semi-closed breathing systems;
(2) Provides for equalization with diving bell environments to extend depth range capabilities;
(3) Provides the ability to change inner counterlung volumetric change rates relative to ambient pressures by applying a pressure or vacuum bias to the pressure sensing devices prior to the dive. This allows for the use of mixed gases in specialized diving applications that would not be usable otherwise;
(4) Provides for easy identification of pressure sensor leaks or miscalibrations;
(5) Provides for easy identification of counterlung leaks;
(6) Provides for easy identification of counterlung contaminants; and
(7) Provides for safe inspired oxygen fraction levels even if the counterlung proportioning mechanism or depth sensor(s) fail.
Accordingly, it is an object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which is more efficient in gas usage as compared to prior art counterlungs.
It is another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides the most efficient use of gas possible in a system that is keyed to respiratory minute volume while still maintaining tight inspired oxygen fractions associated with prior art passive addition semi-closed breathing systems.
It is yet another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for equalization with diving bell environments to extend depth range capabilities.
It is still another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides the ability to change inner counterlung volumetric change rates relative to ambient pressures by applying a pressure or vacuum bias to pressure sensing devices prior to the dive, allowing for the use of mixed gases in specialized diving applications not otherwise usable with prior art devices.
It is even still another object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for easy identification of pressure sensor leaks or miscalibrations.
It is a further object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for easy identification of inner counterlung leaks.
It is still a further object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for easy identification of counterlung contaminants.
It is still a further object of the present invention to provide a variable volume ratio compound counterlung as part of a passive addition semi-closed circuit rebreather which provides for safe oxygen fraction levels even if the counterlung proportioning mechanism and/or depth sensor(s) fail.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.